<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style>
#one{font-size:17px;}
#two{font-size:25px;}
</style>
</head>
<body>
<p id="one">ID选择器one</p>
<p id="two">ID选择器two</p>
</body>
</html>